Adjust app for dark theme

[raimund-schluessler]

Currently the app looks quite broken when using the dark theme:

[raimund-schluessler]

@skjnldsv What can I do to provide inverted (white) icons for the app?

[skjnldsv]

You need to switch to css variables.
Do no use scss colours variables, do not use fixed colours (like #fff) :)

For the icons they should be automatically inverted on cache clear if you use the svg api to register your icons.
https://github.com/nextcloud/server/blob/master/core/css/functions.scss#L67

[raimund-schluessler]

Fix for the vue branch is in #219.

[raimund-schluessler]

Fixed with #194.